A conventional Liquid Crystal Display device (LCD) performs the display based on a spatial color mixing principle, on which a color filter is generally configured. However, in a new field sequential LCD, the display is performed based on a time color mixing principle, that is to say, the three primary colors of the light, i.e. Red (R), Green (G) and Blue (B), are separated along the time axis, and quick switching among pixels of each primary color as time goes on, to display a color image on a screen.
When the field sequential LCD displays a color image, it should be generally noted is the problem about synchronization, coexistence and occupation between the flashing time of a backlight and the scanning time of a gate line on a liquid crystal substrate. In a common existing field sequential LCD, the flashing starting time of the backlight is selected to be after the last line scanning of each sub-frame is finished, and the flashing ending time may be before or after the first line scanning of the next frame is started. No matter which flashing time manner is adopted, there is always color mixing phenomenon. Taking the gate lines scanning along the screen from up to down as an example, when the backlight is turn on, the liquid crystal molecules in the upper portion of the screen have already deflected sufficiently, and the optimal color display can be obtained by turning on the backlight at this time. However, there is not enough response time for the liquid crystal molecules in the lower portion of the screen, and therefore deviation of the color display will be caused in the current frame; and on the contrary, this contributes to the color display of the next frame to a certain extent due to the asynchronization in the response time. This is referred to as the color mixing problem in the field sequential LCD display. If the flashing ending time of the backlight is selected to be before the starting of the first line scanning of the next frame, then the flashing time of the backlight and the scanning time of the gate line do not coexist totally. Referring to FIG. 1, FIG. 1 is a schematic diagram showing the non-coexisting relation between the flashing time of the backlight and the scanning time of the gate line. As shown in FIG. 1, the oblique line indicates the process of scanning the gate lines from the first line to the Nth line, and the rectangles indicate the backlights flashing with different colors. As shown in the Figure, in the case that the frame rate is fixed, if the flashing time of the backlight is too short, the lightness of the emergent light is not enough; and if the scanning time of the gate line is too short, the charging performed on the pixel electrode is not enough and gray scale is distorted. If the flashing ending time of the backlight is selected to be after the starting of the first line scanning of the next frame, the flashing time of the backlight and the scanning time of the gate line coexist partly or mostly, and this will aggravate the above mentioned color mixing phenomenon.
To solve the problem of the color mixing in the field sequential LCD display, a new field sequential LCD, i.e. a Frame buffer (Fb) field sequential LCD is provided. Referring to FIG. 2 and FIG. 3, FIG. 2 is a schematic diagram of a pixel structure of a common field sequential LCD; and FIG. 3 is a schematic diagram of a pixel structure of an Fb field sequential LCD. The pixel structure of the common field sequential LCD includes a Thin Film Transistor (TFT), a storage capacitor Cst and a holding capacitor Clc, in which the gate of the TFT is connected to a gate line, the source thereof is connected to a data line, and the drain thereof is connected to the storage capacitors Cst, and the storage capacitor Cst and the holding capacitor Clc are connected in parallel and are respectively connected to the ground. Compared with the common field sequential LCD, the Fb field sequential LCD further includes a TFT and a frame buffer capacitor Cfb. The pixel structure of this Fb field sequential LCD includes a TFT1, a TFT2, a frame buffer capacitor Cfb, a storage capacitor Cst and a holding capacitor Clc, in which the gate of the TFT1 is connected to a gate line, the source thereof is connected to a data line, and the drain thereof is connected to the source of the TFT2 and an end of the frame buffer capacitor Cfb, and the other terminal of the frame buffer capacitor Cfb is connected to the ground; and the gate of the TFT2 is connected to a Video Synchronization signal (Video Sync, VS), the drain thereof is connected to the storage capacitors Cst, and the storage capacitor Cst and the holding capacitor Clc are connected in parallel and are respectively connected to the ground.
Referring to FIG. 3, in the gate line scanning, the TFT2 is cut off under the control of the video synchronization signal, and in this case, the gray scale voltage signal of the present frame is still applied to the pixel electrode and the display is performed normally, that is to say, the gray scale voltage signal of the next frame that will be input into the pixel electrode is stored in the frame buffer capacitor Cfb. After one frame has been scanned, all the TFT2s conduct under the control of a high-pulse of the video synchronization signal shared by the whole screen, the voltage signal stored in the frame buffer capacitor Cfb enters into the pixel electrode via the channel of TFT2, and all of the TFT2s are cut off after the pixel electrode has been charged. Then the next frame starts to be scanned by the gate lines. This scanning will not affect the electric potential of the pixel electrode because the TFT2 is cut off, meanwhile normal display of the current frame is being performed on the pixel electrode.
FIG. 4 is a schematic diagram of coexisting relation between the scanning time of the gate lines and the flashing time of the backlight in an Fb field sequential LCD. Referring to FIG. 4, the oblique line indicates the process for scanning the gate lines from the first line to the Nth line sequentially, and in this case, TFT1 in each pixel conducts and the signal is inputted sequentially through the date line to charge the frame buffer capacitor Cfb. It can be seen from the Figure that, the scanning time of the gate line and the flashing time of the backlight overlap completely, and the occupation does not exist, so that the charging of the pixel electrode and the flashing of the backlight are sufficient. Therefore, the Fb field sequential LCD can solve the problem of the distribution of the scanning time of the gate line and the flashing time of the backlight, so that the problem of color mixing can be solved.
However, the Fb field sequential LCD also has its own disadvantages. Electric potential analysis will be performed on the pixel electrode of the Fb field sequential LCD hereinafter. Referring to FIG. 3, it is assumed that the electric potential of the pixel electrode of the current frame is V1, and a voltage signal V2 of the next frame is input to the frame buffer capacitor Cfb by the gate line scanning, in this way, when the TFT2 is turn on, the electric potential on the pixel electrode will be V1′, and the following formula can be obtained:V1(Clc+Cst)+V2Cfb=V1′(Cfb+Cst+Clc)  (1)
the following two formulas can be obtained by deducing according to the formula (1):
                              V          1          ′                =                              V            1                    +                                                    (                                                      V                    2                                    -                                      V                    1                                                  )                            ⁢                              C                fb                                                                    C                fb                            +                              C                st                            +                              C                lc                                                                        (        2        )                                                      V            2                    -                      V            1            ′                          =                                            (                                                V                  2                                -                                  V                  1                                            )                        ⁢                          (                                                C                  st                                +                                  C                  lc                                            )                                                          C              fb                        +                          C              st                        +                          C              lc                                                          (        3        )            
During the process of displaying on the Fb field sequential LCD, the Gamma curve of the common driving IC requires an one-to-one correspondence relation between the pixel electrode voltage and the output voltage, that is to say, it is desired that when obtaining a specific pixel electrode voltage, there is only one specific value of the output voltage that corresponds to the pixel electrode voltage. The formula (3) is represented as that: when V2 and V1′ are determined, the difference between V2 and V1′ should be a fixed value (in other words, the difference between V2 and V1′ is a function only dependent on V1′). However, in the formula (3) that is deduced from the pixel electrode structure of the Fb field sequential LCD, when V2 and V1′ are determined, the difference between V2 and V1′ varies with the variance of V1, rather than a fixed value. That is to say, in the case that the same V2 is output, the practical voltage on the pixel electrode will be affected by V1, which be presented on the image as the current frame with the afterimage of the last frame. That is to say, the afterimage will always exist, so as to reduce the image display quality.